In the control of systems having rotating drive shafts, torque and speed are the fundamental parameters of interest. Therefore, the sensing and measurement of torque in an accurate, reliable and inexpensive manner has been a primary objective of workers for several decades. With the relatively recent development of prototype electric power steering systems in which an electric motor driven in response to the operation of a vehicle steering wheel controls the production torque by control of the supply current thereto, the need for a torque sensing apparatus which can accurately detect a torque produced by a steering shaft has been highlighted. Although great strides have been made, these remains a compelling need for inexpensive torque sensing devices which are capable of continuous torque measurements over extended periods of time despite severe environments and operating conditions.
Previously, torque measurement was accomplished using contact-type sensors directly attached to the shaft. One such sensor is a "strain gauge" type torque detection apparatus, in which one or more strain gauges are directly attached to the outer peripheral surface of the shaft and a change in resistance caused by strain is measured by a bridge circuit or other well known means. However, contact-type sensors are relatively unstable and of limited reliability due to the direct contact with the rotating shaft. In addition, they are very expensive and are thus commercially impractical for competitive use on vehicle steering systems.
More recently, non-contact torque sensors of the magnetostrictive type have been developed for use with rotating shafts. For example, U. S. Pat. No. 4,896,544 to Garshelis discloses a sensor comprising a torque carrying member, with an appropriately ferromagnetic and magnetostrictive surface, two axially distinct circumferential bands within the member that are endowed with respectively symmetrical, helically directed residual stress induced magnetic anisotropy, and a magnetic discriminator device for detecting, without contacting the torqued member, differences in the response of the two bands to equal, axial magnetizing forces. Most typically, magnetization and sensing are accomplished by providing a pair of excitation or magnetizing coils overlying and surrounding the bands, with the coils connected in series and driven by alternating current. Torque is sensed using a pair of oppositely connected sensing coils for measuring a difference signal resulting from the fluxes of the two bands. Unfortunately, providing sufficient space for the requisite excitation and sensing coils on and around the device on which the sensor is used has created practical problems in applications where space is at a premium. Also, such sensors appear to be impractically expensive for use on highly cost-competitive devices such as vehicle steering systems.
The output signals of prior art non-contact magnetoelastic torque transducers arise as a result of changes in a magnetic property of a member which is so positioned as to be mechanically stressed, in a usefully correlative manner, by the torque of interest. In all such prior art devices the magnetic property effectively sensed is a permeability .mu., of one form or another. This can be understood from the fact that the output signals of these devices are derived from a magnetic flux density B of a flux which arises in response to an excitation field H with B=.mu.H. While .mu. is clearly alterable by the stress and hence by the transmitted torque, its actual value for any particular stress is largely dependent on both intrinsic and structural properties of the magnetoelastically active material forming the member as well as on its temperature. Moreover .mu. is also strongly dependent on H, in a manner that is neither linear nor monotonic. The effective field H is itself sensitive to the amplitude and frequency of the electric currents from which it is generally derived as well as the distribution of permeances of the associated magnetic circuit. Temperature effects on coil resistance, air gap dimensions, leakage flux associated with permeabilities of yokes and other ancillary portions of the magnetic circuit, dielectric constants of parasitic capacitances between windings and other conductive elements and other factors as well, can all have significant influence on the sensed value of B independently of variations in torque. The basic weakness in this prior art approach to magnetoelastic torque transducers is thus seen to be that the sensed quantity, i.e., B, has a large and complex dependence on many variables and a comparatively small dependence on torsional stress with the undesirable result that the sensed variations in B do not unambiguously indicate a variation in torque.
Attempts to overcome this problem with prior art devices employ constructions providing two distinct B dependent signals, having equal quiescent values but opposite responses to torque, with means for combining the two signals differentially; the idea being to reject common mode variations in B while doubling the sensitivity to changes associated with the torque. The requirement for a zero output signal with zero applied torque demands great care in establishing precise symmetry in the two B sensors and precise equality in both the quiescent .mu. in the two regions of the member being probed and in the exciting fields. The complexities associated with realizing the sought for benefits of these constructions, in the sensor portion itself as well as in the associated electronic circuits required for providing temperature compensating excitation currents and signal conditioning, increases both the cost and the size of a complete transducer while also generally reducing its adaptability, maintainability and reliability.